1. Field of the Invention
The present invention relates to a resulfurized austenitic stainless steel with improved machinability. This steel may be used in the field of machining at high cutting speed, in the field of bar turning, etc.
2. Discussion of the Background
For one skilled in the art, it is understood that high-speed machining of austenitic stainless steels means the use of cutting speeds greater than 500 m/min. The speeds which can be used on a steel are, for example, determined by turning tests with tools having coated-carbide tips, which tests, designated Vb.sub.15/0.15, consist in determining the speed for which the flank wear is 0.15 mm after machining for 15 minutes. Above this speed, it is not possible to machine without risk, while below this speed, machining on an industrial scale is possible.
A resulfurized austenitic stainless steel with improved machinability is known from European Patent No. 403,332, incorporated herein by reference. This document describes a process in which it is proposed, in order to improve the machinability, to introduce, into a steel having the following general composition:
less than 0.15% of carbon, PA1 less than 2% of silicon, PA1 less than 2% of manganese, PA1 less than 3% of molybdenum, PA1 between 7 and 12% of nickel and PA1 between 15 and 25% of chromium, PA1 a quantity of sulfur in a proportion lying between 0.1 and 0.4% and associated with calcium and oxygen having contents greater than 30.times.10.sup.-4 % and 70.times.10.sup.-4 %, respectively, the calcium and oxygen contents being such that the Ca/O ratio is between 0.2 and 0.6. PA1 carbon&lt;0.1% PA1 silicon&lt;2% PA1 manganese&lt;24 PA1 nickel, from 7 to 12% PA1 chromium, from 15 to 25% PA1 sulfur, from 0.10 to 0.55% PA1 copper, from 1 to 5% PA1 calcium&gt;35.times.10.sup.-4 % PA1 oxygen&gt;70.times.10.sup.-4 %, PA1 a sulfur content of between 0.20 and 0.40, more preferably between 0.25 and 0.35%; PA1 a copper content between 1.2 and 3, more preferably between 1.4 and 1.8%, PA1 the composition comprises less than 3% molybdenum.
In this document, the desired aim is the formation, with manganese and, in a lower proportion, with chromium, of a manganese chromium sulfide (Mn,Cr)S which, in the form of specific inclusions, causes solid lubrication of the cutting tool during machining operations.
It is also specified that sulfur has an unfavorable effect on corrosion resistance but that, despite this, a chosen aim is to introduce, into a resulfurized steel, inclusions of aluminosilicate oxides of lime which are usually associated with inclusions of sulfides.
Such an austenitic steel has good machinability properties in the conventional range of cutting speeds, that is to say less than 500 m/min in turning. The steel has associated inclusions composed of aluminosilicate-type oxides which preferentially coat sulfide inclusions. These inclusions are larger and more deformable than sulfide-only inclusions. The effect of the so-called solid lubrication of the cutting tool is improved thereby. However, the steel described in the cited document has a drawback. This is because sulfur reduces the properties of the steel from the standpoint of cold deformation, stress cracking, etc., for example when drawing or wire-drawing.